sigh. Thanks Grammar nazi. "With what do you wipe your ass with", would that be better? Why don't you school me, cuz omg I sux0r with my gremmer.

The original question was OK. It's acceptable to finish with a preposition nowadays, has been for 90 years, but it's a rule people bring up to try and make themselves look intelligent. The with is slightly superfluous, but not offensive.

A bear and a rabbit are taking a shit in the woods.
The bear says to the rabbit, "hey, rabbit. Does shit stick to your fur?"
"No, why?" says the rabbit.
The bear picks up the rabbit and wipes his ass with it.